Power amplifiers are known in the art. One application of power amplifiers is the use of a variable gain power amplifier to produce amplitude modulation in an AM transmitter or in a polar transmitter. In these configurations, a power amplifier can be required to change its output level over a wide range. Depending on the system requirements, the amplifier may be required to produce an accurate output level or an accurate gain between input amplitude and output amplitude. An amplifier may be required to keep a nearly constant phase offset between input signal and output signal. It may also be desirable for an amplifier to have capability to be power efficient at all output levels.
FIG. 1 is a diagram of a typical prior art variable gain amplifier 100, such as might be used in a polar transmitter or AM transmitter. Amplifier 100 includes output stage 102 and driver stage 101. Output stage 102 produces a controlled gain or a controlled output level, such as by using control circuit 103. An RF input signal is provided to an input of driver stage 101, which produces an output signal that is provided to an input of output stage 102, which produces output signal 104. By controlling the gain of output stage 102, the amplitude of output signal 104 can be controlled.
When a coupling 105 exists between the input and output of output stage 102 which is independent of the controlled gain, such as a capacitance of a transistor used to implement output stage 102, metallization capacitance, capacitance due to some other circuit used in conjunction with output stage 102, or other typical capacitances or couplings, undesired system behavior can occur. One common cause of this coupling is gate-drain or base-collector capacitance in an amplifying transistor used to implement output stage 102. Other detrimental couplings 105 from input to output can also occur, such as resistive coupling, inductive couplings, or series and/or parallel combinations of such couplings.
FIG. 2 is a diagram 200 showing an exemplary output amplitude response of prior art variable gain power amplifier 100 containing a coupling 105 across the variable gain output stage. Waveform 201 depicts the output amplitude of output signal 104 of output stage 102 versus the control applied to the output stage. Waveform 202 depicts a desired response of output amplitude versus control applied to output stage 102. The desired response may be linear over a wide range of control, including at small amplitudes. Waveform 201 of amplifier 100 can be linear over a range 204 of relatively large amplitudes. At lower amplitudes, coupling 105 can cause waveform 201 to deviate from the desired response. Coupling 105 can cause the amplifier to produce an output amplitude 203 even when the desired response from the control is to have zero amplitude. This can occur if output stage 102 is producing no gain for this control, while output amplitude 203 is caused by coupling 105. In this case, it can be impossible to control output stage 102 to produce an amplitude of output signal 104 that is lower than output amplitude 203.
FIG. 3 is a diagram 300 showing an exemplary output phase response of prior art variable gain power amplifier 100 containing a coupling 105 across the variable gain output stage. Waveform 301 depicts the output phase of output signal 104 of output stage 102 versus the control applied to the output stage. Waveform 302 depicts a desired response of output phase versus control applied to output stage 102. The desired response may be for a constant output phase over a wide range of control, including at small amplitudes. Waveform 301 of amplifier 100 can be nearly constant over a range 304 of relatively large amplitudes. At lower amplitudes, coupling 105 can cause waveform 301 to deviate from the desired response. This deviation can occur if the component of the output signal due to coupling 105 has a different phase than the desired output signal due to output stage 102. For output amplitudes where the signal component due to coupling 105 is comparable or greater in magnitude than the signal component due to output stage 102, the phase of output signal 104 can move away from the desired response towards the phase of the leakage signal 303.
FIG. 4 is a diagram 400 showing an exemplary power consumption of prior art variable gain power amplifier 100. Waveform 401 depicts the power consumption of output stage 102 versus the control signal applied to output stage 102. The power consumption of output stage 102 can vary with the control signal, typically increasing as the gain or output amplitude increases. Waveform 402 depicts the power consumption of driver stage 101. At low gains or output amplitudes, the power consumption due to the driver stage can be comparable to or even greater than the power consumption due to output stage 102. At sufficiently low output amplitudes, the signal at the output of driver stage 101 may be stronger than output signal 104, so that the driver amplifier can be amplifying the signal only to have output stage 102 attenuate it to a lower level.